I have a simple project with a close deadline.

I am trying to light up 5 LEDs as part of the project. The first 4 light up in sequence fine, and if I light up just the last one it lights up fine, but if all 5 are lit at once the very last LED is very dim. I swapped out cables, the pin on the Arduino, resistor, LED.

What would explain that? Input amperage? Too strong a resistor?

I have a 330 ohm resistor in between each LED and ground. I am using one of the neat resistors from SparkFun that packages up the 5 330 ohm resistors together, with a common ground wire. I bought two and tried swapping it out with no change.


Here is my code:

/* Blink without Delay

const int startButtonPin = 6;
const int reedSwitchPin = 7;
const int ledPins[] = {8,9,10,11,13};
const int numLeds = 5;
const long interval = 1000;           // interval at which to blink (milliseconds)
const int revolutionsPerStage = 15;

int debounceDelay = 500; // for switches--interval to ignore short button presses/reed switch open/close
long lastDebounce = 0;
int buttonState = LOW;
int mapStage = 0;

unsigned long previousMillis = 0;        // will store last time LED was updated

void setup() {
  // set pin modes
  for (int thisPin = 0; thisPin < (sizeof(ledPins)/sizeof(int)) - 1; thisPin++)  {
    pinMode(ledPins[thisPin], OUTPUT);      
  pinMode(startButtonPin, INPUT);

  digitalWrite(reedSwitchPin,HIGH); // activate the pull-up resistor

// Main program loop
void loop()
  buttonState = digitalRead(startButtonPin);

  // wait for button press to begin program proper. we won't get back to this state except on first boot or if program finishes
  if (buttonState == HIGH) {

void trackProgress() {
  int blinkState = LOW;
  unsigned long currentMillis = millis();
  unsigned int numRevolutions = 0;

  do {
    digitalWrite(ledPins[mapStage], HIGH);

    // blink the current goal LED at constant interval specified earlier
    if(currentMillis - previousMillis >= interval) {
      // save the last time you blinked the LED
      previousMillis = currentMillis;  
      blinkState = !blinkState;
      digitalWrite(ledPins[mapStage], blinkState);
  } while (mapStage < numLeds);

// perform a simple welcome sequence to indicate we are ready to start pedalling
void chasingLEDs() {
   // turn LEDs on in sequence, then back off
   for (int i = 0; i < numLeds; i++){
     digitalWrite(ledPins[i], HIGH);

   for (int j = numLeds; j > 0; j--) {
     digitalWrite(ledPins[j], LOW);

   // flash the LEDs twice
   for (int k = 0; k < 2; k++) {
     for (int l = 0; l < numLeds; l++) {
         digitalWrite(ledPins[l], HIGH);
     for (int m = 0; m < numLeds; m++) {
         digitalWrite(ledPins[m], LOW);

1 Answer 1


I found the problem, it was a bug in my code. I had found code to read the array size online but it was off by 1.

Instead of:

for (int thisPin = 0; thisPin < (sizeof(ledPins)/sizeof(int)) - 1; thisPin++)  {
    pinMode(ledPins[thisPin], OUTPUT);      

When I changed it to thisPin < numLeds, it worked correctly. The last pin was simply set to be an input pin instead of an output pin.


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